Since an Nb3Al wire produced by rapid-heating, quenching and transformation (RHQT) process which has initially been developed for the use in an NMR gives priority to good superconducting joint performance, and Nb3Al interfilamentary barrier is formed from Nb which maintains superconductivity even in a magnetic field of less than 1 T, there is a disadvantage in that Nb3Al filaments are electromagnetically coupled together to cause low magnetic-field instability.
For removing the disadvantage, instead of Nb, Ta which similarly belongs to a high melting-point metal (refractory metal) and which exhibits normal conductivity in a magnetic field of less than 1 T, was used as a barrier material, but there were problems in that the suppression of the low magnetic-field instability was impossible at a temperature lowered to 2 K which is the operation temperature of an accelerator magnet, and a wire breakage often occurred during the wire drawing process of a precursor wire, and the like.
FIG. 1 is a cross-sectional view of a precursor wire for an Nb3Al wire produced by rapid-heating, quenching and transformation process, in which an interfilamentary barrier is formed from Nb, which has conventionally been developed for the use in the NMR. An outer cover (skin) 1 and dummy filaments 8 constituting a center portion of a wire cross-section are individually formed from Nb which belongs to the high melting-point metal.
An Nb/Al composite filament region 3 is prepared by a jelly roll method in which an Nb foil and an Al foil are stacked and wrapped around an Nb core (see patent document 1 (Japanese Patent No. 4005713)) or a rod-in-tube method in which a number of Al cores or Al alloy cores are dispersed in an Nb matrix (see patent document 2 (Japanese Patent No. 1888312)).
In the RHQT process, a precursor wire, which is reel-to-reel moving, is subjected to rapid heating and quenching treatment by allowing a current to flow through the precursor wire so that the precursor wire is heated to about 1,950° C., and passing the resultant precursor wire through a melted Ga electrode bath kept at about 80° C., which serves also as a coolant, to convert the Nb/Al composite filament regions 3 to bcc phase supersaturated solid solution filaments comprising an Al supersaturated Nb. Then, the resultant wire is subjected to additional heat treatment at 800° C. for 10 hours to change (transform) the crystal structure of the filaments from the bcc phase to an A15 phase, obtaining Nb3Al superconducting filaments (see patent document 3 (Japanese Patent No. 2021986)).
Nb can maintain the superconducting state in a magnetic field of less than 1 T at 4 K, and therefore when the outer cover 1 and an interfilamentary barrier 4 are formed from Nb, the joint resistance of the joint portions between a number of superconducting wires, which are required in the application to an NMR magnet, can be markedly reduced.
On the other hand, in a magnetic field region of less than 1 T, individual Nb3Al filaments are electromagnetically coupled together through the Nb interfilamentary barriers 4, so that an effective superconducting filament diameter is disadvantageously increased.
Therefore, the multi-filamentary wire cross-sectional structure having dispersed a number of thin superconducting filaments, which has been proposed for suppressing the magnetic instability, does not properly function in the low magnetic field region. Actually, with respect to a RHQT treated Nb3Al wire, the precursor wire in which the interfilamentary barrier is formed from Nb, in magnetization curves shown in FIG. 2 as measured at two coolant temperatures expected in the application to a superconducting magnet (i.e., at a liquid helium temperature 4 K and a superfluid helium temperature 2K), magnetic instability called a flux jump occurs such that the magnetic flux line (fluxoid) rapidly penetrates into a sample (the magnetization is rapidly reduced).
For suppressing the low magnetic-field instability, electromagnetically decoupling of Nb3Al filaments is considered. For achieving this, it is necessary to change the material for interfilamentary barrier from Nb, which maintains superconductivity even in a low magnetic field, to another metal which is in a normal conductivity state in a magnetic field. Such a metal having normal conductivity in a magnetic field is required to have at least the following properties:
(1) excellent wire drawability (plastic deformability) such that the wire drawing of the precursor wire is not inhibited;
(2) excellent mechanical properties even at high temperatures such that the metal serves as a mechanical reinforcement to prevent the precursor wire from suffering a creep rupture during the reel-to-reel heat treatment which is conducted by passing a current through the moving wire (self-heating) at 1,900° C. or higher for 5 seconds or less for converting the Nb/Al composite filament regions to bcc phase supersaturated solid solution filaments; and
(3) non-reactivity with the Nb/Al composite filament regions during the reel-to-reel heat treatment, that is, properties such that the metal not only does not inhibit the formation of a supersaturated solid solution after quenching but also does not form a solid solution together with the finally obtained Nb3Al to cause the deterioration of superconductivity.
As an example of the most candidate material which satisfies the above requirements, there can be mentioned Ta. Ta has a superconducting critical temperature (4.4 K) which is slightly higher than a boiling temperature of liquid He (4.2 K) under atmospheric pressure, but has a small temperature margin (difference between a critical temperature and a coolant temperature), and therefore Ta has characteristics such that transition to the normal conducting state occurs immediately after a magnetic field is applied to Ta.
Further, Ta has the following advantageous properties:
(1) cold wire drawability comparable to that of Nb;
(2) a melting point higher than that of Nb and a creep strength rather higher than that of Nb at about 1,950° C.;
(3) non-reactivity with the Nb/Al composite regions during the reel-to-reel heat treatment; and the like.
In the production of the Nb3Al precursor wire produced by the RHQT-process in which the interfilamentary barrier is formed from Ta, the wire drawability was a little poor, as compared to that of the precursor wire in which the interfilamentary barrier is formed from Nb, and hence the precursor wire suffered breaking of wire caused from the interfilamentary barrier portion as a starting point three times during the wire drawing, so that the unit length (length per continuous wire) of the resultant wire was disadvantageously small, but the wire drawing was able to be done so that the final wire diameter became 1.35 mm.
FIG. 3 illustrates a transverse cross-sectional structure of the obtained precursor wire. The precursor wire was subjected to rapid heating and quenching treatment to convert the Nb/Al composite filament regions 3 to a bcc phase supersaturated solid solution, and then the resultant wire was further subjected to transformation heat treatment at 800° C. for 10 hours to convert the bcc phase supersaturated solid solution to A15 Nb3Al. As can be seen from the magnetization curves shown in FIG. 4, a flux jump (low magnetic-field instability) was able to be suppressed at 4 K as expected.
However, when the coolant temperature was lowered to 2 K, a flux jump, though slightly, occurred again. From this result, it has been found that the use of the Nb3Al wire produced by rapid-heating, quenching and transformation process in which the interfilamentary barrier is formed from Ta is difficult in the operation at 2 K which is the temperature of a superconducting magnet expected in a high energy particle accelerator or the like.
The reason for this is considered that the temperature margin of the Ta interfilamentary barrier is large at 2 K so that the Nb3Al filaments are electromagnetically coupled together.
When the interfilamentary barrier is changed to Ta, at least an effect of suppressing the low magnetic-field instability at 4 K can be obtained without using expensive Ta in the outer cover (skin) and center dummy filaments. Specifically, as seen in the example shown in FIG. 5, relatively inexpensive Nb may be used in the outer cover 1 and center dummy filaments 8.
With respect to the lowering of the wire drawability of the precursor caused due to the employment of Ta in the interfilamentary barrier, for suppressing this, a method has been proposed in which the Nb/Al composite filament region is first covered with a first barrier layer formed from Nb and further covered with a second barrier layer formed from Ta (see patent document 4 (Japanese Patent Application No. 2009-241004)).
By this method, the precursor wire can be prevented from suffering breaking of wire during the wire drawing while surely suppressing the low magnetic-field instability at 4 K; however, the state of the precursor wire in which the adjacent Nb3Al filaments are connected through a superconducting material of Ta remains unchanged, and therefore the occurrence of a flux jump at 2 K is unavoidable.
As mentioned above, the RHQT-process Nb3Al multi-filamentary wire is produced by subjecting the bcc phase supersaturated solid solution, which has been formed by a high-temperature heat treatment at about 1,950° C. and quenching, to additional heat treatment (transformation heat treatment) at 700 to 900° C. to transform the solid solution to an A15 Nb3Al compound.
A transformation temperature is relatively low, and hence A15 compound crystal grains obtained after the transformation are finely divided particles, which cause critical current density of Nb3Al to be high.
In the RHQT-process, there is required a high-temperature short-time heat treatment performed at a temperature even higher than the melting point of Cu used as a stabilizer, and therefore, unlike precursor wires for commercial superconducting wires, the precursor wire cannot have a Cu stabilizer incorporated to a surface thereof in advance.
Instead, a step is needed in which the precursor wire is subjected to rapid heating and quenching treatment, and then subjected to Cu ion plating for improving the adhesion while removing an oxide film on a wire surface (Nb or Ta), and subsequently a large amount of Cu is incorporated to the surface of the precursor wire by electroplating. However, the Cu ion plating/electroplating step has a drawback in that the cost for the wire is increased.
On the other hand, as a method for reducing the cost required for the incorporation of a stabilizer, there has been proposed an internal stabilization method characterized in that a stabilizer is isolated from the Nb/Al composite filament regions by a high melting-point metal to prevent the stabilizer fused during the high-temperature short-time heat treatment from reacting with the Nb/Al composite filament regions to form a non-superconducting ternary compound, so that the stabilizer is initially contained in the cross-section of the wire (see patent document 5 (Japanese Patent No. 4386306)).
In the internal stabilization method, the Cu stabilizer (or Ag stabilizer) filaments having a high electric conductivity are individually covered with Ta (or Nb, Ta), and can be disposed in arbitrary positions of the wire cross-section in principle, but, actually, as shown in FIG. 6, the stabilizer filaments are concentrated and disposed in the middle of the wire cross-section.
According to the combination of types of the materials for the outer cover, interfilamentary barrier, internal stabilizer filaments, and covering layer for the internal stabilizer filaments, various internal stabilized Nb3Al precursor wires shown in FIGS. 6 to 10 are present.
When the high-temperature short-time heat treatment at about 1,950° C. for converting the Nb/Al composite filament regions to the bcc phase supersaturated solid solution is performed, the Cu (or Ag) stabilizer is fused, but Cu (or Ag) fused liquid is enclosed in a Ta (or Nb, Ta) covering tube. The Ta (or Nb, Ta) covering tube serves as a diffusion barrier, making it possible to prevent a reaction of Cu (or Ag) with the Nb/Al filament regions. Like bulkheads of an oil tanker, Ta (or Nb, Ta) layers of a honeycomb form serve as walls for enclosing the Cu (or Ag) fused liquid, further preventing the wire from suffering creep deformation in heating at high temperatures.
After quenched to 500° C. or lower, the Cu (or Ag) fused liquid is solidified and serves as a stabilizer. With respect to the precursor wire obtained in the same manner as in the conventional precursor wire containing no internal stabilizer filaments in the cross-section (FIGS. 1, 3, and 5) where the Nb/Al composite filament regions would be converted to the bcc phase supersaturated solid solution by quenching and then subjected to additional heat treatment at 800° C. for 10 hours to transform the bcc phase supersaturated solid solution to Nb3Al, a critical current density calculated by dividing a critical current by a sectional area of an Nb3Al phase was the same as that of the precursor wire containing no internal stabilizer filaments.
When the internal stabilizer is Cu, the interfilamentary barrier formed from Ta suppresses the occurrence of a flux jump at 4 K, but the occurrence of a small flux jump at 2 K is unavoidable.
Further, the precursor wire suffered a wire breakage caused from the interfilamentary barrier portion as a starting point during the wire drawing of precursor wire, as illustrated in FIGS. 3 and 5, in which no internal stabilizer is contained but the interfilamentary barrier is formed from Ta.
On the other hand, when the internal stabilizer is Ag, which is not reacted with Nb or Ta at a high temperature (1,950° C.), a rapid heating and quenching treatment can be made, irrespective of whether the interfilamentary barrier is formed from Nb or Ta. However, when the interfilamentary barrier is formed from Nb (critical temperature: 9 K), which exhibits superconductivity even in a magnetic field, the Nb3Al filaments are electromagnetically coupled together, causing marked low magnetic-field instability at both temperatures of 4 K and 2 K. When the interfilamentary barrier is formed from Ta (critical temperature: 4 K), like the case where the internal stabilizer is Cu, the occurrence of a small flux jump at 2 K is unavoidable, and the precursor wire inevitably suffers a wire breakage during the wire drawing.